The heat deflection temperature or heat distortion temperature (HDT, HDTUL, or DTUL) is the temperature at which a polymer or plastic sample deforms under a specified load. This property of a given plastic material is applied in many aspects of product design, engineering, and manufacture of products using thermoplastic components.

The Heat Distortion Temperature is determined by the following test procedure outlined in ASTM D648. The test specimen is loaded in three-point bending in the edgewise direction. The outer fiber stress used for testing is either 0.455 MPa or 1.82 MPa, and the temperature is increased at 2 °C/min until the specimen deflects 0.25 mm. This is similar to the test procedure defined in the ISO Limitations that are associated with the determination of the HDT is that the sample is not thermally isotropic and, thick samples in particular, will contain a temperature gradient. The HDT of a particular material can also be very sensitive to stress experienced by the component which is dependent on the component’s dimensions. The selected deflection of 0.25 mm (which is 0.2% additional strain) is selected arbitrarily and has no physical meaning.

Thermal simulations of a system will show temperatures that will be encountered by a specific component of that system. Knowing what temperature that a specific component will have to endure during use will allow the determination of the best material for that application.

• Example: One of two materials may be used for Component A of a system, acrylic or polycarbonate. Component A will have to endure temperatures of 120 °C during use. Polycarbonate (HDT=140 °C) will not deform at 120 °C but acrylic (HDT=90 °C) would deform. Polycarbonate would be used for Component A in this case.

Vicat softening point or Vicat hardness is the determination of the softening point for materials that have no definite melting point, such as plastics. It is taken as the temperature at which the specimen is penetrated to a depth of 1 mm by a flat-ended needle with a 1 square mm circular or square cross-section. For the Vicat A test, a load of 10 N is used. For the Vicat B test, the load is 50 N.

Standards to determine Vicat softening point include ASTM D 1525 and ISO 306, which are largely equivalent.^[1]